1. Field of the Invention
The present invention relates to light-emitting diode (LED) based signboards. In particular, the present invention relates to increasing both functionality and reliability of such LED-based signboards.
2. Discussion of the Related Art
Light emitting diodes (LEDs) produce most of the active images shown on modern advertising structures. A large number of LEDs (e.g., hundreds of thousands to millions) are used on a typical signboard to produce a multicolored image. Thus, the reliability of both the pixels formed from groups of LEDs and their associated electronics is an important design consideration. Thus, it is important to be able to detect and to handle LED failure, incurring only a minimal down time.
In a typical signboard, the LEDs are arranged in small groups, with each group providing a picture element (pixel) in the image being displayed. Each pixel is capable of displaying a wide range (“gamut”) of colors. Typically, each pixel1 is made up of three kinds of LED. Each “kind” of LEDs may consist of a single LED, or a serially connected string of LEDs, providing a specific color of light (“primary color”). Popular LEDs provide red, green and blue lights. Light of a wide variety of colors and intensities may be produced from each pixel by properly controlling the intensity of light emitted from each kind of LED. The intensity of light emitted from each LED kind is controlled by the electrical current flowing through the LED. In addition, the human psycho-visual system is insensitive to light intensity changes that are more rapid than about 100 Hz. For these reasons, the typical driver for an LED, or for a string of serially connected LEDs, is made up of a current source that is pulse-modulated to produce two states: i.e., either having no current or a current of a reference value. The modulation rate is chosen so that the waveform has essentially no energy present below about 100 Hz. A duty cycle may be selected so that the average value of the current waveform over time provides the required light intensity from the LEDs. The desired duty cycle is stored in a counter that is preset by digital circuitry to correspond to the relative intensity desired from a particular kind of LED (e.g., red-emitting) within a pixel. The reference value Iref of the current is such as to provide a desired brightness for the entire image display consisting of many pixels. 1In the present description, a pixel may include one or more LEDs provided within a locality of the signboard to appear to a distant viewer as an illuminated point on the display. The LEDs forming the pixel may be addressed and programmed as a single unit, or as separate individual units.
For convenience in construction, installation and maintenance, a typical signboard organizes its pixels in groups, with each group being housed in a common structure or module. A group typically consists of hundreds to thousands of pixels. Sometimes, each group is further subdivided into many parts each consisting of a few to a few tens of pixels. However, since each color in each pixel must be controlled independently of all others, large amounts of data must flow to each group of pixels whenever a change is made in the image displayed on the advertising structure. To show a motion picture on such a structure would require the ability to handle a huge data flow rate. Contemporary signboards use many parallel wires to transfer the data and additional wires for control and monitoring functions. Consequently, a large number of connectors are required for interconnecting components. The cost and reliability of the connectors, the cost of manufacture and the cost of maintenance all suggest that alternative methods for accomplishing the interconnections are desirable.
As signboards are large outdoor structures, their exposed faces become dirty and must be cleaned to preserve the quality and appearance of the images shown. Additionally, particularly for structures exposed to strong sunlight, the faces may be also exposed to significant heat loads. Therefore, cleaning the faces and controlling the thermal environment can prolong the life and reduce repair and maintenance costs.
The entire set of colors that a light-emitting display is capable of showing is called its color gamut, which is a function of all primary colors that the light-emitting elements can produce. Typically, a set of LEDs may provide a gamut which produces images exceeding the gamut capability of the display system that generates or processes the images. As a result, the gamut available on a signboard may not be fully utilized. The images shown thus may not have the attention-capturing or aesthetic impact that would be possible if the gamut were more effectively utilized.
Further, in humans, color perception changes with the ambient lighting condition. A color perceived in a bright background looks different when the background brightness changes, so that some signboards may be difficult to read or an image appears to be of the wrong or unnatural colors under certain lighting conditions. Accordingly, a method for compensating for perceived color shift due to ambient light is desired.